Pressure fluid actuated tools such as pile drivers are normally driven by steam or compressed air. These devices generate two types of noise: noise from the hammer impact and noise from exhaust leaving the hammer actuating chamber. The latter is created by the release of the exhaust and is commonly referred to as "jet noise". It is created by high velocity air turbulence as the exhaust air leaves the actuating chamber and flows through nonaerodynamic turns in the passages leading to the atmosphere.
Recently, concern has been raised over noise levels in the environment which has resulted several laws and ordinances prescribing certain noise level codes. Prominent examples include ordinances in very large cities, such as New York City, where the urban din from construction, traffic, etc. can sometime prove intolerable.
Many commercial reactive-type exhaust mufflers for pressure actuated tools reduce the noise produced thereby but at cost of generating substantially high back-pressure due to the drastic slowing of the high velocity exhaust flow, especially those having baffles within the exhaust chamber. See for example U.S. Pat. No. 2,128,742 to Fuehrer and U.S. Pat. No. 2,561,726 to Cherain.
Alternatives to these mufflers include absorptive types which have either elongated or round housings. These designs provide desirable acoustical characteristics by providing high absorption while exhibiting minimal reverberation which would otherwise contribute noise by transforming the muffler housing into a noise source. However, they suffer disadvantages from a practical standpoint in that their shapes are inconvenient for attaching to pile driving apparatus since the muffler is exposed to severe jolts and vibrations.